Simple Interest Calculator
By Santaji GadepatilinUpdated October 27, 2026
Total amount vs Time (years) — other inputs held at your current values.
How to use this calculator
Fill in any three of principal, rate, time and interest (or total amount). The rest is calculated. For example, to find the rate you need, press × on the rate and enter the principal, time and interest you want to earn.
The formula
I = P × r × t A = P + I
- P — principal (amount borrowed or invested)
- r — annual interest rate as a decimal (6% = 0.06)
- t — time in years
- A — total amount at the end
Worked example
50,000 invested at 6% simple interest for 3 years:
- I = 50,000 × 0.06 × 3 = 9,000
- A = 50,000 + 9,000 = 59,000
Simple vs compound interest
For the same 50,000 at 6%:
| Years | Simple interest total | Compound (yearly) total |
|---|---|---|
| 1 | 53,000 | 53,000 |
| 3 | 59,000 | 59,550.80 |
| 10 | 80,000 | 89,542.38 |
| 20 | 110,000 | 160,356.77 |
The gap widens over time. See the compound interest calculator for growth with compounding.
Where simple interest is used
- Some short-term personal and car loans
- Certain bonds and treasury bills
- Late-payment interest on invoices
- Quick estimates before using a more detailed model
Frequently asked questions
What is the simple interest formula?
Simple interest = P × r × t, where P is the principal, r the yearly rate as a decimal and t the time in years.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is also charged on interest already added, so it grows faster over time.
Can I enter time in months?
Yes. Switch the Time unit to months, days or weeks and the calculator converts it to years.